1. Field of the Invention
The present invention relates to a semiconductor apparatus, and more particularly relates to a semiconductor apparatus for converting an analog signal into a digital data.
2. Description of the Related Art
In recent years, regulations with regard to the safety of a vehicle have been successively strengthened in Japan and the United States. According to the TREAD law (The Transportation Recall Enhancement, Accountability and Document Act) executed in North America, a new car that will be sold from 2006 onward will be obligated to mount a tire pressure monitoring system (hereinafter also referred to as TPMS). For this reason, the technique has been currently developed for mounting a sensor inside a tire to measure a tire pressure and an air temperature. This technique is related to the system that mounts a sensor unit in a valve portion of each tire and individually monitors all four wheels. Thus, this technique has an advantage that the monitoring with a high precision, the monitoring of the tire pressure during the stopping and parking of the car, and the like can be attained.
The foregoing sensor unit typically includes: a sensor; an amplifying apparatus for amplifying an output of the sensor; an AD converter for performing an AD conversion on an output of the amplifying apparatus; and a micro computer (a processing unit) for computing a sensor output data based on an output of the AD converter.
A TPMS represented by the TPMS having the foregoing sensor units uses a pressure sensor to measures a tire pressure of a tire. When the pressure sensor is used to monitor a state of a target (such as an tire pressure of a tire in the TPMS), at first, an amplifying circuit amplifies a weak voltage that is a potential difference between two analog signals outputted from the pressure sensors, and generates an amplification voltage. Next, an AD converting circuit converts the amplification voltage (an analog value) into an AD conversion value (a digital value) to enable the amplification voltage to be processed by the micro computer. Then, the AD converting circuit supplies the AD conversion value to the micro computer. The micro computer executes a predetermined computing process based on the AD conversion value to obtain a tire pressure data. Typically, the range of an input voltage that can be converted by the AD converting circuit is the range from 0 V (a ground potential of the AD converting circuit) to a reference voltage (Vref) applied to the AD converting circuit. For this reason, even if a voltage less than 0 V or a voltage more than the reference voltage (Vref) is supplied to the AD converting circuit, the AD converting circuit cannot output a correct AD conversion value. In short, with regard to the input voltage less than 0 V, the AD converting circuit outputs a constant digital value (typically, 0) corresponding to 0 V. On the other hand, with regard to the input voltage more than the reference voltage, the AD converting circuit outputs a constant digital value (typically, a maximum value that can be represented by a bit number of the AD conversion) corresponding to the reference voltage.
In the TPMS, the outputting of the monitored result with a high precision and reliability is required. Conventionally, in order to improve the reliability of the TPMS, a method is used in which the AD converting circuit having a high resolution is employed in the system. As one method of detecting the tire pressure with the high resolution, there is a method in which the weak voltage outputted by the pressure sensor is amplified by the amplifying circuit whose amplification factor is high and converted into the digital value by the AD converting circuit. As another method, there is a method that increases the number of bits in the AD conversion and consequently converts the weak input voltage into the digital value with the high resolution. Japanese Laid Open Patent Application (JP-A-Heisei, 5-248975) discloses a gain adjusting circuit of an amplifier for amplifying a signal of a pressure sensor. When an amplification factor of the amplifying circuit is made higher, the input voltage to the AD converting circuit is made higher. On the other hand, as mentioned above, the maximum value of the input voltage on which the AD converting circuit can perform the AD conversion is determined by the reference voltage. Thus, in order to correctly perform the AD conversion on the amplified input voltage, the reference voltage is required to be higher. With this conversion into the high voltage, when the AD converting circuit is built in the micro computer, it becomes difficult to make the operation voltage of the micro computer itself lower than the reference voltage.
On the other hand, when the method that increases the number of the bits in the AD conversion to improve the resolution is used as mentioned above, it is possible to avoid the reference voltage from being made higher. However, the increase in the number of the bits in the AD conversion increases the circuit scale of the AD converting circuit and finally leads to the increase in a chip area and the expensive cost. Depending on a case, it becomes difficult to build the AD converting circuit in the micro computer.
If a use environment temperature of the tire is largely changed, the tire pressure is also largely changed. Also, it is desired to be able to recognize a sign of a tire blowout by detecting the slight change of the tire pressure. Therefore, the AD converting circuit used in the TPMS is required to have both the performance for surely converting the output voltage of the pressure sensor, which is changed over a wide range, into the digital value, and having the high resolution at which the slight change in the output voltage of the pressure sensor can be detected.